Solved

# for nieto

Posted on 2000-04-30
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can u explain the code u post for the calendar?
how that equation works?
can u explain the number u used?
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Question by:danieldaniel_2000
• 2

LVL 22

Expert Comment

Is this for me?

If so, I'm not sure what it refers to.
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Author Comment

here it is:
can u tell me how u get those number for the equation?

From: nietod
Date: Monday, February 08 1999 - 04:10PM PST
For example,
// The date in dd/mm/yyyy.
int dd=1;
int mm=1;
int yy=1900;

int jdn;
char *weekday[]={"Tuesday",
"Wednesday",
"THursday",
"Friday",
"Saturday",
"Sunday",
"Monday"};

if (mm < 3)
{
mm += 12;
yy -= 1;
}
jdn = dd+1720996+(mm+1)*306/10+yy*365 + yy/4 - yy/100 + yy/400;

printf("%s\n",weeday[jdn%7]);

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LVL 22

Accepted Solution

nietod earned 5 total points
That is not something I invented.  It is a relatively well known formula.  It calculates an integer serial date.  That is, for any date it calculates a unique integer value that is one more than the value produced by the previous date.  This can be used to calculate the number of days between two dates.

The numbers were chosen so that when the numbers entered are rounded to an integer they yield the right result    For example,
306/10 is the average number of days in a month, when you do mm+1*306/10 you get the number of days into the year at the start of the specifid month  Add dd to that (which is the first item) and you get the number of days into the year etc.   You add on yy*365 to get the number of  days at the start of the year from year 0 (there was no actual year 0, but...) The three figures at the end are to correct for leap years.  i.e. dividing the year by 4 gives the number of leap years since year 0, which gives the number of extra leap days that have been accumlated by the specified date.  (Or would have accumulated, if leap years were performed since year "0", they didn't start untile the 1600's so the foruma isn't accurate before then.)  - yy/100 corrects for the fact that a year that is divisible by 100 is not a leap year, even though it is divible by 4.  and so on.
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